feat: share shopping lists via a public link

Mirrors the existing recipe public-share pattern (/r/[id], gated by a
visibility flag, no auth required) — shopping lists previously only
supported private per-user collaborator invites (email + viewer/editor
role), with no way to hand someone a link who isn't already a member.

- shoppingLists.isPublic (owner-only toggle, same access-control pattern
  already used for renaming/deleting the list).
- New public /s/[id] route added to PUBLIC_PATHS — read-only aisle-grouped
  view with a signup CTA for logged-out visitors, styled after /r/[id].
- The existing collaborator-invite dialog (ShareShoppingListButton) now
  also has a "Public link" toggle + copy-link button at the top, so
  there's one Share entry point for both private and public sharing.

Verified locally: toggling public via the real dialog UI persists
correctly (confirmed via DB + a fresh curl PATCH to rule out a client
race in my own test script), and the public link loads with zero auth
for a logged-out browser context.

apps/web/components/shopping-lists/share-shopping-list-button.tsx

@@ -2,7 +2,7 @@
 
 import { useState } from "react";
 import { useTranslations } from "next-intl";
-import { UserPlus, X } from "lucide-react";
+import { UserPlus, X, Link2, Copy, Check } from "lucide-react";
 import { toast } from "sonner";
 import {
   Dialog,
@@ -13,6 +13,7 @@ import {
 } from "@/components/ui/dialog";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
+import { Switch } from "@/components/ui/switch";
 import {
   Select,
   SelectContent,
@@ -21,6 +22,7 @@ import {
   SelectValue,
 } from "@/components/ui/select";
 import { Badge } from "@/components/ui/badge";
+import { Separator } from "@/components/ui/separator";
 
 type Role = "viewer" | "editor";
 
@@ -38,9 +40,10 @@ interface Member {
 
 interface Props {
   listId: string;
+  initialIsPublic: boolean;
 }
 
-export function ShareShoppingListButton({ listId }: Props) {
+export function ShareShoppingListButton({ listId, initialIsPublic }: Props) {
   const t = useTranslations("shoppingLists");
   const ts = useTranslations("shareDialog");
   const tCommon = useTranslations("common");
@@ -50,6 +53,42 @@ export function ShareShoppingListButton({ listId }: Props) {
   const [members, setMembers] = useState<Member[]>([]);
   const [loading, setLoading] = useState(false);
   const [inviting, setInviting] = useState(false);
+  const [isPublic, setIsPublic] = useState(initialIsPublic);
+  const [savingPublic, setSavingPublic] = useState(false);
+  const [copied, setCopied] = useState(false);
+
+  async function togglePublic(checked: boolean) {
+    setSavingPublic(true);
+    const previous = isPublic;
+    setIsPublic(checked);
+    try {
+      const res = await fetch(`/api/v1/shopping-lists/${listId}`, {
+        method: "PATCH",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ isPublic: checked }),
+      });
+      if (!res.ok) {
+        setIsPublic(previous);
+        toast.error(ts("publicLinkToggleFailed"));
+      }
+    } catch {
+      setIsPublic(previous);
+      toast.error(ts("publicLinkToggleFailed"));
+    } finally {
+      setSavingPublic(false);
+    }
+  }
+
+  async function copyLink() {
+    const url = `${window.location.origin}/s/${listId}`;
+    try {
+      await navigator.clipboard.writeText(url);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    } catch {
+      toast.error(ts("copyLinkFailed"));
+    }
+  }
 
   async function fetchMembers() {
     setLoading(true);
@@ -130,6 +169,25 @@ export function ShareShoppingListButton({ listId }: Props) {
             </DialogDescription>
           </DialogHeader>
 
+          <div className="flex items-center justify-between gap-3 rounded-lg border p-3">
+            <div className="flex items-center gap-2 min-w-0">
+              <Link2 className="h-4 w-4 text-muted-foreground shrink-0" />
+              <div className="min-w-0">
+                <p className="text-sm font-medium">{ts("publicLinkTitle")}</p>
+                <p className="text-xs text-muted-foreground">{ts("publicLinkDescription")}</p>
+              </div>
+            </div>
+            <Switch checked={isPublic} disabled={savingPublic} onCheckedChange={(v) => { void togglePublic(v); }} />
+          </div>
+          {isPublic && (
+            <Button type="button" variant="outline" size="sm" className="w-full" onClick={() => void copyLink()}>
+              {copied ? <Check className="h-4 w-4" /> : <Copy className="h-4 w-4" />}
+              {copied ? ts("linkCopied") : ts("copyLink")}
+            </Button>
+          )}
+
+          <Separator />
+
           <div className="flex gap-2 mt-2">
             <Input
               type="email"